Photoredox-Catalyzed Radical Coupling of C7-Chloromethyl-Substituted Thiazolino Ring-Fused 2-Pyridones with Quinoxalinones

We have developed an Ir(PPy)3 photoredox-catalyzed cross-coupling reaction that allows installation of quinoxalinones at the C7 position of thiazolino ring-fused 2-pyridones (TRPs) under mild conditions. The methodology tolerates various substituted quinoxalinones and biologically relevant substituents on the C8 position of the TRP. The TRP scaffold has large potential in the development of lead compounds, and while the coupled products are interesting from a drug-development perspective, the methodology will be useful for developing more potent and drug-like TRP-based candidates.

T he thiazolino ring-fused 2-pyridone (TRP) scaffold has been a foundational element in the development of diverse biologically active compounds.The substituents on the pyridone ring influence the potency and determine which target an analogue is active toward.For example, compound A (Figure 1A) has broad spectrum bactericidal activity toward, among others, methicillin-resistant S. aureus (MRSA), vancomycin-resistant enterococci (VRE), and streptococcal species. 1 Compound B (Figure 1A) is capable of inhibiting the aggregation of Aβ(1−40) amyloid fibers, 2 and compound C (Figure 1A) belongs to a class that prevents biofilm formation of Mycobacterium tuberculosis (Mtb) and restores the effect of isoniazid (INH) on INH-resistant Mtb strains. 3,4Compound D (Figure 1A) has antivirulent properties toward uropathogenic E. coli 5 and was developed for fluorescent labeling.In the development of new analogues, the substituent on the C7 position has been especially important for activity, and finding new methods to functionalize this position is therefore important to access more potent and drug-like derivatives.While current methods already exist (Figure 1B), none have made use of photocatalyzed processes which offers unique reactivity and allow introduction of substituents that are unattainable with other methods.Exploring photocatalyzed methods with TRPs is therefore interesting as late-stage modification is already challenging with this complex scaffold containing multiple functionalities which complicates selective modifications.In this work, we have developed a photoredoxcatalyzed cross-coupling reaction (Figure 1C) that allows installation of substituted quinoxalinones at the C7 position.The reaction proceeds under mild conditions, making use of single-electron transfer (SET) from Ir(PPy) 3 .Historically, bulkier groups such as naphthalene at the C7 position have generated the most successful analogues.The similarity in size to naphthalene and the heterocyclic nature of quinoxalinones therefore make their fusion with TRPs an interesting pairing.−9 While the generated compounds may possess interesting biological activity, the developed methodology is a gateway toward other, previously unexplored, C7-substituents on the TRP scaffold.

■ CONDITION SCREENING
The reaction was initially carried out with a cyclopropylsubstituted chloro-TRP (2a, 1.0 equiv) and an unsubstituted quinoxalinone (1a, 1.2 equiv) dissolved in MeCN (9.4 mL/ mmol 2a) (Table 1, entry 1).The mixture was irradiated with 455 nm LEDs with a luminous flux of 130 lm for 300 min, which generated the desired product 3aa (36%, entry 1).Intrigued by the results, we investigated if the reaction could be made more efficient by introducing a photoredox catalyst.Screening of different photoredox catalysts (entries 2−6) revealed that Ir(PPy) 3 was the most suitable, facilitating the reaction in 30 min with 56% yield.Interestingly, the yields when using organic photoredox catalysts (entries 3−5) were similar to when no catalyst was added, suggesting that their presence had no effect on the reaction.Solvents were then screened (entries 7−10), which showed that the reaction proceeded best in MeCN but also worked well in DCM and DCE.Based on the hypothesized mechanism, we expected that the addition of a base would be favorable; however, the addition of K 2 CO 3 (1.0 equiv, entry 11) gave similar results as without a base, and the addition of TEA (1.0 equiv., entry 12) resulted in a lower yield.Using 2.0 equiv of 1a (entry 13) instead of 1.2 equiv resulted in a small increase in yield, from 56% to 64%, but this increase did not justify its continued use.
The necessity of degassing the reaction mixture prior to irradiation was also investigated, which showed that the reaction is able to proceed with oxygen present but results in a lower yield (entry 14).The reaction also proceeded well with an increase in reaction time when scaled to 501 mg of 2a, giving product 3aa in 48% yield after recrystallization from MeOH (entry 15).A series of control experiments was also performed, confirming that the reaction does not proceed efficiently when irradiated at 395 nm, does not proceed at all without light or with only heating, and requires dry conditions (Supporting Information, Table S1).

■ QUINOXALINONE SCOPE
With the proper conditions in hand, the scope of the reaction was tested, first with differently substituted N-methyl quinoxalinones (Scheme 1).Introduction of one or two  The Journal of Organic Chemistry methyl groups on the quinoxalinone was well tolerated, giving the expected products 3ba and 3ca in 57% and 69% yields, respectively.The reaction also proceeded well with a 6,7dimethylated quinoxalinone without a methylated amide nitrogen, giving 3da in 51% yield.Changing the substitution from 6,7-dimethyl to 6,7-difluoro while keeping the amide nitrogen free resulted in a complex mixture with traces of 3ka.
Substitution with strongly electron-withdrawing groups such as NO 2 and CN required longer reaction times before the TRP starting material was consumed (up to 1 h) and gave products 3la and 3ma, respectively, with inseparable impurities.
Replacing the electron-withdrawing group with an amine gave product 3na with inseparable impurities.Halogen substituents (F, Cl, and Br) were tolerated, giving the corresponding products 3ea−3fa in 15−36% yield.Trifluoromethyl-substituted quinoxalinone was also tolerated, giving product 3ja in 31% yield.From these experiments it is observed that substitution on the C5 position of the quinoxalinone does not influence the yield significantly.The yields of reactions with C6-and C7-substituted quinoxalinones are more affected by the electronic properties of the substituents compared to the C5-substituted ones as σdonating substituents increase the yield while σ-withdrawing substituents decrease yields compared to nonsubstituted quinoxalinone.

■ TRP SCOPE
The effects of different substituents on the C8 position of the TRP were investigated using 1c as the quinoxalinone coupling partner (Scheme 2).The size of the C8 substituent on the TRP did not appear to influence the yield strongly as both mtrifluoromethylphenyl-substituted and nonsubstituted TRPs gave their respective products, 3cb and 3cc, in 54% yield.

■ MECHANISTIC INVESTIGATION
To gain insight into the mechanism of this transformation, cyclic voltammetry and Stern−Volmer experiments were conducted on 2a (Figure 2).
Cyclic voltammetry showed three irreversible oxidation peaks at 1.55, 1.35 and 1.02 V and two irreversible reduction peaks at −1.80 and −2.37 V. Knowing the redox potential of Ir(PPy) 3, 10 the theoretical driving force for the irreversible reduction of 2a by Ir(PPy) 3 * was calculated using the Rehm− Weller equation. 11This gave a negative Gibbs free energy difference (ΔG) of −0.12 eV, suggesting a positive driving force for the reaction.The Stern−Volmer experiment showed a linear relationship between the reduction in fluorescence intensity of Ir(PPy) 3 and the increasing concentration of 2a, suggesting that 2a is able to quench Ir(PPy) 3 *.
To investigate the presence of a radical intermediate, a radical trapping experiment was carried out under the optimized conditions between 2a and 1c with the addition of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) (1.2 equiv) to the reaction mixture (Scheme 3).
This gave exclusively the TEMPO-coupled product (3oa), which supports the hypothesis that the reaction proceeds through a radical intermediate.
Based on the combined results from the mechanistic investigation, the radical trapping experiment and reported Scheme 2. Substrate Scope of TRPs with 1c Volmer plot with the quenching rate constant (k q ) calculated from the linear fit using τ 0(Ir(PPy)3) = 1.9 μs 10 under oxygen-free conditions.

Scheme 3. Radical Trapping Experiment with TEMPO
The Journal of Organic Chemistry mechanisms of radical couplings with quinoxalinones in the literature, 12−17 we propose the following mechanism for the transformation (Scheme 4).
The photocatalyst is excited by the 455 nm light and undergoes oxidative quenching with the C7-methyl chloride TRP (I), generating a benzyl-type radical (II) and a chloride ion.The TRP radical then undergoes radical addition to the imine carbon of the quinoxalinone (III), resulting in a Ncentered radical (IV).This radical species can then undergo a 1,2-hydride shift to a more stable tertiary C-centered radical (V). 15,16The photocatalyst is then regenerated through SET from V to the catalyst, generating a positively charged TRP− quinoxalinone-coupled species (VI) that gives the final product 3aa after deprotonation.

■ CONCLUSION
We have developed a method to introduce quinoxalinones on the C7 position of substituted TRPs using photoredox catalysis.This method provides a new and fast approach to C7 functionalization of TRPs and has provided insight into the photochemical properties of C7-chloromethyl-substituted TRPs.With this, a new tool for C7 functionalization has been discovered which can be used in the future for incorporation of other unexplored radical acceptors in the pursuit of more potent and drug-like TRP-based drug candidates.

■ EXPERIMENTAL SECTION
General Information and Data Collection.All reagents and solvents were used as received from commercial suppliers without further purification.All of the necessary reactions were carried out in dry solvents under a nitrogen atmosphere.Reaction progress was monitored on aluminum-based silica gel TLC plates (median pore size 60 Å, fluorescent indicator 254 nm) and detected with UV light at 254 and 366 nm.Automated flash column chromatography was performed using a Biotage Isolera One system and purchased prepacked silica gel cartridges (BiotageSfar, duo 60 μm). 1 H, 13 C, and 19 F NMR spectra were recorded on a Bruker AVANCE III 400 MHz spectrometer (101 MHz 13 C, 376 MHz 19 F) with a BBO-F/H Smart probe at 298 K unless otherwise stated.All spectrometers were operated by Topspin 3.5.7.Structural assignments were made with additional information from gHSQC and gHMBC experiments.LC-MS was conducted on a Micromass ZQ mass spectrometer using ES+ ionization.HRMS was performed on an Agilent mass spectrometer with ESI-TOF (ES+).FTIR spectra were acquired by pressing the solid sample onto the diamond window of an attenuated total reflectance (ATR) cell (Golden Gate, a single bound diamond window) and then measuring the spectrum with a resolution of 4 cm −1 over the 600−4500 cm −1 range at a forward/reverse scanning rate of 10 kHz on a Bruker Vertex 70/V instrument.Absorption spectra were acquired in solution using 10 mm path length quartz high-precision cell cuvettes and a UV−vis spectrophotometer (UV5, Mettler Toledo) with background subtraction for the solvent (MeCN).Excitation and emission spectra were recorded in solution using 3 mm path length quartz high precision cell cuvettes with a HORIBA Jobin Yvon spectrofluorometer (FluoroMax-3).Cyclic voltammetry was performed in a 30 mL glass cell using a using a modulab potentiostat (Solartron Analytical, AMETEK).The working electrode was a glassy carbon electrode, polished using a cotton polishing cloth with slurries of progressively finer alumina particles (1, 0.3, and 0.05 μm) purchased from BUEHLER, IL.The quasireference electrode was a silver wire, and the counter electrode was a Pt sheet.
Synthesis of Quinoxalinones (1a−1m).. 17,18 The di-, mono-, or nonsubstituted phenylene-1,2-diamine (1 equiv) was added to an appropriately sized round-bottom flask followed by absolute ethanol (2.3 mL/mmol).Ethyl glyoxalate (1.2 equiv) was then added as a 50% (w/w) solution in toluene, and the resulting mixture was stirred at 90 °C in an oil bath for 1 h and then at room temperature overnight.The mixture was then cooled in ice water, and the resulting solid was filtered off using vacuum filtration.The solid was washed with ice cold absolute ethanol on the filter until the liquid passing through was colorless.After extensive drying on the filter, the solid was transferred to a round-bottom flask and further dried at 50 °C under reduced pressure for 2 h.The crude material was used, without further purification, in either methylation or directly in the coupling reaction.
Methylation of Quinoxalinones.The dried, crude, quinoxalinone (1 equiv) was mixed with K 2 CO 3 (1.2equiv) and suspended in DMF (4.6 mL/mmol) before MeI (1.6 equiv) was added.The flask was then capped with a septum, and the mixture was stirred at room temperature and monitored by TLC (80% EtOAc in heptane).Upon completion, the mixture was diluted with deionized water (4.6 mL/ mmol) and transferred to a separatory funnel using EtOAc (23 mL/ mmol).The aqueous phase was extracted with 2 × EtOAc (11 mL/ mmol), and the combined organic phases were washed once with sat.NH 4 Cl (aq) (7 mL/mmol) and then once with brine (10 mL/mmol).The washed organic phase was then dried over Na 2 SO 4 and filtered into a round-bottom flask before the solvent was removed using rotary evaporation.
Synthesis of Quinoxalinone-Coupled Pyridones (3aa−3oa).Quinoxalinone 1a−1n (1.2 equiv), C7-methyl chloride-pyridone 2a− 2g (1.0 equiv), and Ir(PPy) 3 (2 mol %) were added to an oven-dried photoreaction vial which was then sealed with a Biotage septum cap for microwave reaction vials.The vial was then evacuated and backfilled with N 2(g) 3 times on a Schlenk line.Dry MeCN (9.4 mL/ mmol pyridone) was then added, and the mixture was degassed by purging with N 2(g) for 5 min.The mixture was then stirred at 22 °C with water cooling while irradiating with 455 nm LEDs with an input power of 10 W and output luminous flux of 130 lm for 30 min to 1 h (for setup see Supporting Information Figures S.1−S.5).Reactions were monitored with TLC (80% EtOAc in heptane).When all of the TRP was consumed, the solution was transferred to a separatory funnel using EtOAc (10 mL) followed by deionized water (10 mL) and brine (2 mL).The aqueous phase was extracted with EtOAc (3 × 10 mL), and the combined organic phases were dried over Na 2 SO 4 and filtered into a 100 mL round-bottom flask.The solvent was then removed using rotary evaporation.

Data Availability Statement
The data underlying this study are available in the published article and its Supporting Information.

Figure 1 .
Figure 1.(a) Examples of active analogues based on the TRP scaffold.(b) Previous methods to diversify the C7 position.(c) Photoredoxcatalyzed cross-coupling between a C7-methyl chloride-substituted TRP and substituted quinoxalinones.

Table 1 .
Optimization of the Conditions for Photoredox-Catalyzed Cross-Coupling between 1a and 2a a Isolated yields.bInitially irradiated for 16 h at their λ max , 527 nm.c 2 equiv of 1a.dNo degassing.eDone on 501 mg scale of 2a, and the product was recrystallized from MeOH.Scheme 1. Substrate Scope of 2a with Quinoxalinones